This invention relates to field-induced injection currents across insulating layers, and more particularly, to rapidly obtained high quantum yields thereof when the insulating layer is in contact with a biasing means and a photoconductive layer and the photoconductor is struck by suitable light.
Light is physically viewed as possessing the characteristics of wave motion and energy particles. The characteristic of an energy particle is generally attributed to the photon quantum of light. Generally speaking, when a photon quantum of light strikes a photoconductive material, one pair of charge carriers constituting a negative charge and a positive charge is created. Typically, one of the charge carriers of the pair of charge carriers moves in the photoconductive material struck by the photon quantum of light while the other remains substantially in the location of creation. The negative charge carrier is generally referred to as an electron while the positive charge carrier is generally referred to as a hole. It has been found that when light strikes a photoconductive material, one photon quantum of light is required to generate one pair of charge carriers. Thus, the maximum quantum efficiency, expressed as number of pair of charge carriers created per photon quantum of light, has a maximum value of 1. Typically, the quantum efficiency is less than 1.
In order to obtain photoconductive gain greater than unity, it has been generally felt necessary that Ohmic contact e.g., a reservoir of charge at the metal-photoconductor interface, be present. It was further generally felt that with a blocking contact to the photoconductor, e.g., a non-ohmic contact with a Schottky energy barrier at the metal-photoconductor interface, photoconductive gains greater than unity could not be achieved. See, for example, "Photoconductive Gain Greater than Unity in CdSe Films with Schottky Barriers at the Contacts", R. R. Mehta and B. S. Sharma, J. Appl. Phys., Vol. 44, No. 1, January, 1973. According to this article, the authors were able to achieve a photoconductive gain greater than unity with gold electrodes in contact with the photoconductor wherein the gold contacts were determined to be non-ohmic with a Schottky energy barrier between the electrode and the photoconductor and wherein the radiation utilized was bandgap radiation of the photoconductor. No insulating layer was deliberately inserted between the gold electrode and photoconductor.
Conduction through a physical, electrically insulating barrier is reported in "Thermally Assisted Tunneling in Dielectric Films", G. G. Roberts and J. I. Polanco, Phys. Stat. Sol. (a), 1, 409 (1970). In the latter article, the authors reported findings in the characteristic relationship between current flow in, and voltage applied to, an insulating organic layer sandwiched between two electrodes. No photoconductive layer is utilized and no mention is made of gain photocurrent.
Conduction through a semi-conductor layer adjacent a few-atoms-thin layers of insulating materials is theoretically presented in "The Physical Review B", F. Schmidlin, 1, 4, pages 1583-1587 (1970).
U.S. Pat. No. 3,732,429 discloses the use of an inorganic insulating layer in contact with a photoconductor in order to obtain a higher dark impedance in conjunction with a liquid crystalline layer. All three layers are sandwiched between a pair of electrodes.
Copending application Ser. No. 489,285, filed July 17, 1974, U.S. Pat. No. 3,958,207 discloses a method for obtaining gain photocurrent across an insulating layer in contact with a photoconductive layer sandwiched between a pair of electrodes. Gain photocurrent and thermal tunneling through the insulating layer are obtained after a short delay when light to which the photoconductive layer is responsive is impinged on the layer while an electrical potential is applied between the electrodes. In order to obtain a gain photocurrent in prompt response to activating radiation a low-intensity light is maintained on the photoconductive layer in the presence of an electrical field. The low-intensity light produces sufficient hole-electron pairs in the photoconductor to cause migration and buildup of a charge at the insulator-photoconductor interface. The buildup of charge is not in itself of sufficient strength to cause gain photocurrent. However, it is sufficiently strong to cause prompt thermal tunnelling through the insulating layer and gain photocurrent when light of a higher intensity strikes the photoconductor.
An apparatus and method for producing gain photocurrent promptly with activating light of a single intensity is desirable.